Quantitative analysis of elemental and molecular species, and often an isotopic ratio of species, is a key interest in many fields of science. For instance, accurate and quantitative determination of elemental and molecular species finds application in environmental, science, material sciences, life science and geology.
A fundamental challenge for accurate and precise quantitative mass spectrometry of molecular and elemental species is the interference between a species of interest and another species having the same nominal mass. One example of a problematic interference is that of isotopologues within a sample having the same nominal mass. For example, in the analysis of methane, 13CH4+, 12CH3D+ and 12CH5+ all have a nominal mass of 17 but an exact mass that differs as a consequence of nuclear mass defect.
In order to permit discrimination between interfering species, e.g. same nominal mass isotopologues, a mass spectrometer having relatively high mass accuracy is necessary. One such device, sold by Thermo Finnigan under the brand name Neptune™, is described in Weyer et al, International Journal of Mass spectroscopy, 226, (2003) p 355-368. The Neptune™ device is a double focusing multiple collector inductively coupled plasma (MC-ICP) mass spectrometer and may be used to determine isotopic fractions of atomic and polyatomic ions. The detector chamber of the mass spectrometer is equipped with a plurality of Faraday collectors. Ions are spatially separated by the mass analyzer in accordance with their mass to charge ratio. Each Faraday collector is precisely aligned with respect to atomic and polyatomic ions of a particular nominal mass. The Faraday collectors are each provided with an entrance slit. In use, the parameters of the mass analyzer are adjusted so that ions of different masses are scanned across the slit. With suitably high resolution, ion species of the same nominal mass but different true masses can be separately detected.
Our co-pending application no. GB1514471.0, filed on even date, describes a double focusing gas isotope ratio mass spectrometer (GIRMS) developed by Thermo Fisher Scientific under the name 253 Ultra™. The device has a multiple collector positioned at the focal plane of a double focusing magnetic sector mass analyser. High, medium and low resolution can be selected automatically using a switchable spectrometer entrance slit. The device is capable of resolutions up to several tens of thousand.
The multiple collector comprises a fixed axial collector which is a dual mode detector having a Faraday cup and a high sensitivity ion counting detector (SEM). The multiple collector also carries 8 moveable detector platforms mounted as 4 platforms on each side of that fixed axial collector. Each moveable detector platform is equipped with a Faraday detector and can also carry a compact discrete dynode (CDD) ion counting detector. In total, the multiple collector can thus carry 9 Faraday detectors (the axial detector plus 8 more, located 4 each side of the axis) and 8 CODs (again, 4 each side of the axial Faraday detector).
FIG. 1 shows an ideal high resolution scan across the slit of a Faraday collector in a double focusing gas isotope ratio mass spectrometer such as the 253 Ultra™ described above. The presence of “steps” at the shoulders of the main peak is analytically interesting since it may permit identification of different isotopologues or other distinct species.
FIG. 2 shows a scan across a Faraday detector slit with a first signal artefact that can sometimes be observed, when the mass spectrometer is operated at high resolutions up to, for example, 40,000. The artefact is labelled 1 in the figure. As may be seen, the artefact is proximal to the shoulder of the peak, where analytically interesting information may be present. Thus the presence of the artefact 1 in FIG. 2 is undesirable.
FIG. 3 shows a high resolution scan across a Faraday detector slit having a second signal artefact, labelled 2 in the figure, which can also sometimes be observed. Again, artefact 2 is found at the end/shoulder of the main peak, and its presence can reduce or completely mask the ability to detect any analytically significant peak information that would otherwise be seen at the peak shoulders.
The present invention seeks to identify and address problems with Isotope Ratio mass spectrometers such as the GIRMS and MC-ICP MS, that result in the various unwanted artefacts described above.